A Fluid handle is a handle to a Fluid object like a DataStore or a SharedObject (DDS). It can be used to represent the object in the system and has the capability to get the underlying object by calling get() on it.
The two major interfaces required to implement a Fluid handle are IFluidHandle and IFluidHandleContext defined in fluidHandle.ts.
IFluidHandle has only one method get that is used to retrieve the underlying object it represents. It also extends IFluidHandleContext.
IFluidHandleContext describes a routing context (another IFluidHandleContext) that has a path to this IFluidHandleContext. When creating a Data Store Handle the route context should be the FluidDataStoreRuntime which knows how to reach the FluidDataStore. Similarly, the FluidDataStoreRuntime's route context should be the ContainerRuntime which knows how to reach it.
For more details on Fluid Handles, check this doc.